THE surprise resignation of Pope Benedict XVI – the first by a pontiff for almost 600 years – has led to calls for his replacement to be the Catholic Church’s first black leader.

In a letter to his Catholic brethren Pope Benedict, 85, said his advanced age and the pace of change in the modern world had left him unable to “adequately fulfil the ministry entrusted to me”.

Tributes have poured in from religious and political figures around the world – but many also expressed their shock at the news.

Some commentators argued that the Roman Catholic Church should take the opportunity to modernise, after eight years under Pope Benedict in which it was accused of being overly conservative, and hobbled by sexual abuse scandals.

There are already calls for the next Pope to be from Latin America, a stronghold for the faith.

Others say it is time for the first African Pope, with some online betting sites putting Cardinal Peter Turkson from Ghana as an early favourite successor.

Cardinal Turkson is the president of the Pontifical Council for Justice and Peace., appointed by Pope Benedict in 2009. At that time, asked if he would like to see a black Pope, he said "if God would wish to see a black man also as pope, thanks be to God” and noted that former UN secretary-general Kofi Annan was also from Ghana.

Cardinal Francis Arinze of Nigeria has also been named as a potential African successor.

Another candidate favoured by betting sites is Canadian cardinal Marc Ouellet, formerly archbishop of Quebec City – even though the 68 year-old head of the Congregation for Bishops was once quoted saying being the pope “would be a nightmare”.

Catholic commentator and writer Mary Kenny told the BBC she looked to see a younger, possibly Latin American candidate as a favourite to take the post.

There has not been a Pope from outside Europe for at least 600 years.

Pope Benedict, formerly Cardinal Joseph Ratzinger, was one of the oldest ever new popes – aged 78 when he took the post in 2005.

He announced his decision in Latin during a meeting of Vatican cardinals on Monday morning, the BBC reported.

He will step down at the end of February, and a conclave will anoint a successor.

Vatican spokesman Greg Burke said to expect a new Pope by Easter, which falls on March 31 this year.

Pope Benedict will have no official role in choosing his successor, and is expected to retire to the papal residence at Castel Gandolfo.

In a statement released by the Vatican, Pope Benedict said he had “repeatedly examined my conscience” before “coming to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministery”.

He said the modern world, “subject to so many rapid changes and shaken by questions of deep relevance for the life of faith”, required a “strength of mind and body” that he had lost in the last few months.

Previous Popes have stayed in the role until their death, despite physical and mental decay, in the belief that their prayer and suffering as they approach the end are a part of their role.

Georg Ratzinger told the BBC his brother was having increasing difficulty walking and that his resignation was part of a "natural process".

He added: "His age is weighing on him. At this age my brother wants more rest."

Pope Benedict said he was “well aware” of the seriousness of his act, and he asked “pardon for all my defects”.

Max Seckler, a theology professor and close friend of Pope Benedict, told the (London) Telegraph: "He suffered a lot under certain things that were part of his role. It is hard to imagine what intrigues he had to deal with in Rome. It burdened him because he is a theologian and noble person.

"He has always been prepared to take bold steps, and this is one of them."

Last year the Pope appeared unsteady in public on some occasions, using a cane to support himself. Recently he appeared to have trouble reading the text of an address he delivered in Rome.

Benedict had hinted in a book of interviews in 2010 that he might resign if he felt he was no longer able to carry out his duties.

One source told the BBC that the Pope’s doctor had told him he was too unwell for international travel.

Italian president Mario Monti said he was “greatly shaken by this unexpected news”.

But he added he deeply respected the decisions which he was sure “has been inspired by the will to serve the Church to the end and to make sure that it will be guided steadfastly in the future.”

British PM David Cameron said the Pope “will be missed as a spiritual leader to millions”, and German Chancellor Angela Merkel said the decision had her utmost respect.

“He is and remains one of the most significant religious thinkers of our time,” she said.

Archbishop of Westminster, Vincent Nichols said the announcement had shocked and surprised everyone.

“Yet, on reflection, I am sure that many will recognised it to be a decision of great courage and characteristic clarity of mind and action,” he said.

Australian Cardinal of the Roman Catholic Church and Archbishop of Sydney, George Pell, said the resignation had come as a surprise to him.

"Pope Benedict has always loved the church and worked to do what was best for her," Cardinal Pell said.

"We thank him for his years of devoted leadership and service and his brilliant teaching.

"We will pray for him as he enters retirement. We shall also pray for the church as she prepares to choose the next successor of St Peter."

But not all paid tribute to the retiring Pope.

Australian Survivors Network of those Abused by Priests (SNAP) issued a statement calling for the next pontiff to be more cooperative with inquiries into abuse claims, saying “victims welcome the resignation of a church official with immense power who has done so little to stop the reign of terror of child rapist priests.

“In the eyes of many victims, Joseph Ratzinger has personally done much to add to the huge number of victims and exponentially increase the suffering of those already harmed.”

And rationalist Professor Richard Dawkins tweeted he felt sorry for the Pope’s “wasted life (with) no sex”.

Popes have always been able to resign in theory, simply by writing a letter of resignation to the College of Cardinals - the Catholic Church body that elects them.

The only legal requirement is that the reasons be made public.

Pope John Paul II reportedly considered resigning in 2000, age 80. Historians have also speculated that Pope Pius XII drew up a resignation letter in case he was kidnapped by Nazis.

However the last pope to actually resign was Pope Gregory XII in 1415, to help resolve a dispute between the three people who claimed to be the Pope.

Before that, in 1294, Pope Celestine V resigned only five months into the job, before going to live as a hermit.

Despite joining Twitter at the end of last year, Pope Benedict did not use the social messaging service to announce his resignation. His last online comment was that “we must trust in the mighty power of God’s mercy”.